Sodium and potassium ion-activated adenosine triphosphatase [(Na++K+)-ATPase] is inserted into the plasma membranes of animal cells and is responsible for the high concentration of potassium and low concentration of sodium in the cytoplasm. The consequent differences in the steady-state concentrations of these cations between the cytoplasm and the extracellular environment provide potential energy employed to maintain cellular volume, to drive the uptake of nutrients, to move water, and to create the resting potential of cells. Conventional electron microscopy, x-ray diffraction and computer-assisted, three-dimensional reconstruction of low-dose micrographs will be employed to investigate the structural organization of (Na++K+)-ATPase and the conformational changes that this molecular assembly undergoes in order to mediate active transport. The structural data will be collected from crystalline specimens of native (Na++K+)-ATPase molecules poised in defined conformational states. We will produce the crystalline specimens both from membrane-bound preparations and from fully active enzyme dissolved in solutions of detergent. Preliminary studies have shown that crystalline patches of (Na++K+)-ATPase molecules can be produced by changing the concentrations of ionic ligands in the medium. In these crystalline specimens, we will focus our attention on the subunit composition of each molecular assembly as well as its exact position relative to the plane of the membrane. Also, the assignment of the locations of the Alpha and Beta subunits in the unit cell will be confirmed by specifically labeling the arrays with antibodies against the Alpha subunit of the enzyme or lectins that bind to the carbohydrate moiety of the Beta subunit and processing the images by Fourier methods. One of the most exciting aspects of these investigations will be our ability to study the molecules of (Na++K+)-ATPase in the frozen hydrated state at liquid nitrogen temperature. These experiments will provide a low-resolution, three-dimensional map of the molecular structure of this enzyme.